Download Table 7.1. Some secondary metabolites derived from

Biology of Fungi
Lecture 8: Fungal Metabolism and Fungal Products
Table 7.1. Some secondary metabolites derived from different pathways and
precursors [after Deacon, 2006]
Precursor
Pathway
Sugars
Metabolites; representative organisms
Few, e.g. muscarine (Amanita muscaria)
kojic acid (Aspergillus spp.)
Aromatic amino acids
Shikimic acid
Some lichen acids
Aliphatic amino acids
Various, including
peptide synthesis
Penicillins (P. chrysogenum, P. notatum)
Fusaric acid (Fusarium spp.)
Ergot alkaloids (Claviceps, Neotyphodium)
Lysergic acid (Claviceps purpurea)
Sporidesmin (Pithomyces chartarum)
Beauvericin (Beauveria bassiana)
Destruxins (Metarhizium anisopliae)
Organic acids
TCA cycle
Rubratoxin (Penicillium rubrum)
Itaconic acid (Aspergillus spp.)
Fatty acids
Lipid metabolism
Polyacetylenes (Basidiomycota fruitbodies
and hyphae)
Acetyl-CoA
Polyketide
Patulin (Penicillium patulum)
Usnic acid (many lichens)
Ochratoxins (Aspergillus ochraceus)
Griseofulvin (Penicillium griseofulvum)
Aflatoxins (A. parasiticus, A. flavus)
Acetyl Co-A
Isoprenoid
Trichothecenes (Fusarium spp.)
Fusicoccin (Fusicoccum amygdali)
Several sex hormones: sirenin, trisporic
acids, oogoniol, antheridiol
Cephalosporins (Cephalosporium and
related fungi)
Viridin (Trichoderma virens)
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Biology of Fungi
Lecture 8: Fungal Metabolism and Fungal Products
u Examples of secondary metabolites:
Q Penicillin, griseofulvin, other antibiotics
Q Pigments such as melanin and carotenoid
Q Plant hormones like gibberellins
Q Pharmaceuticals like ciclosporin A
Q Aflatoxins
Q Ergot alkaloids
u Why secondary metabolism, especially since they hold no apparent selective advantage
(i.e., genes should be lost)?
Q Necessary as escape valve for intermediates of primary metabolic pathways when
growth is restricted
Q Provide a selective advantage that is yet to be made obvious
u Key intermediate pathways and precursors include those shown in Table 7.1 of Deacon
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Biology of Fungi
Lecture 8: Fungal Metabolism and Fungal Products
u Anaplerotic reactions include:
Q Production of oxaloacetate from pyruvate by the addition of carbon dioxide
Q Glyoxylate pathway - a type of short-circuited TCA pathway
l
l
Isocitrate is converted to glyoxylate, which is then converted to malate, which then
forms oxaloacetate.
Oxaloacetate is used to form PEP/sugars
Lysine Biosynthesis
u Lysine is an essential amino acid that is not naturally produced by human and many animals
u Two pathways found in microbes and plants for producing lysine
Q DAP - a-diaminopimleic acid precursor
Q AAA - a-aminoadipic acid precursor
u DAP pathway is used by plants, bacteria, and the Oomycota
u AAA pathway is used by chitin-containing fungi and some euglenids
u Divergence of these pathways is evolutionarily signficant
Secondary Metabolism
u Secondary metabolism refers to a diverse range of metabolic reactions not directly or
obviously involved in normal cellular growth
u Thousands of secondary metabolites have been described by fungi
u Common features of secondary metabolites from fungi include:
Q Tend to be produced at the end of exponential growth or during substrate-limited
conditions
Q Produced from common metabolic intermediates, but use specialized pathways encoded
by specific genes
Q Not essential for growth or normal metabolism
Q Production tends to be genus-, species-, or strain-specific
u Collectively, these commonalities tend to argue for tight regulatory control of secondary
metabolism
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Biology of Fungi, Lecture 8: Fungal Metabolism and Fungal Products
Energy Acquisition
u Fungi use glycolysis and the tricarboxylic acid (TCA) pathways to
Q Generate energy
Q Provide biosynthetic precursors
u Glycolysis
Q Glucose is converted to pyruvic acid
Q Pyruvic acid is a key intermediate
u Fate of pyruvic acid
Q Oxygen present - transported to the mitochondrion, converted to acetyl-coenzyme A
which is then processed through the TCA cycle
Q Oxygen absent/limiting - undergoes fermentation to produce
l
Ethanol (via acetylaldehyde)
l
Lactic acid
u Energy yield
Q Aerobic respiration - 38 potential ATPs produced per glucose molecule; actual yield is
lower
l
Use of intermediates for other reactions
l
Use of NADP/NADPH for re-dox reactions
Q Fermentation - 2 ATP molecules
u Use of alternative terminal electron acceptors
Q Some fungi use nitrate instead of oxygen as terminal electron acceptor
l
Yields potentially 26 ATPs per glucose
l
Lower due to differences in ATP generation in the electron transport chain
Q Considered anaerobic respiration (as opposed to aerobic respiration when oxygen is
used)
Balancing the Pathways
u Intermediates of glycolysis and the TCA pathway are used to make other substances
u How does the cell replace these components to maintain energy production?
Answer: Anaplerotic reactions
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